Before a defibrillator is triggered so as to discharge a pulse of current between the paddle electrodes held against a patient's chest, it is necessary to determine the action of the heart. To do this, electrical signals that represent heart action are obtained from the paddle electrodes or from a set of ECG electrodes, amplified in a signal channel, and displayed as a waveform by an oscilloscope or by some other suitable means. If the displayed waveform indicates that the heart is operating in a random fashion, known as fibrillation, the defibrillator can be discharged, but if the display indicates that the heart is operating cyclically, other types of treatment are generally used. The waveform for each heart cycle is generally comprised of a sequence of positive and negative peaks having different amplitudes. Under the conditions prevailing when a defibrillator is in use, the peak-to-peak amplitude and hence the aspect ratio of the different sequences of signals can change rapidly by such large amounts as to make analysis of the displayed waveform difficult or impossible. In order to keep the aspect ratio within limits that medical personnel are used to interpreting, it is generally necessary to adjust the gain of the signal channel. If this is done manually, it can cause significant delay in the vital resuscitation precedure, especially where there is only one operator. Therefore, it is desirable to provide circuits for automatically controlling the gain. Circuits that use negative feedback to continually adjust the gain cause distortion in the waveform of the heart action that may be particularly disadvantageous when the defibrillator is being used to monitor the patient.